Ribosome-inactivating proteins (RIPs) comprise a class of proteins which is ubiquitous in higher plants. However, such proteins have also been isolated from bacteria. RIPs are potent inhibitors of eukaryotic protein synthesis. The N-glycosidic bond of a specific adenine base is hydrolytically cleaved by RIPs in a highly conserved loop region of the 28S rRNA of eukaryotic ribosomes thereby inactivating translation.
Plant RIPs have been divided into two types. Stirpe et al., FEBS Lett., 195(1, 2):1–8 (1986). Type I proteins each consist of a single peptide chain having ribosome-inactivating activity, while Type II proteins each consist of an A-chain, essentially equivalent to a Type I protein, disulfide-linked to a B-chain having cell-binding properties. Gelonin, dodecandrin, tricosanthin, tricokirin, bryodin, Mirabilis antiviral protein (MAP), barley ribosome-inactivating protein (BRIP), pokeweed antiviral proteins (PAPs), saporins, luffins, and momordins are examples of Type I RIPS; whereas Ricin and abrin are examples of Type II RIPs.
Amino acid sequence information is reported for various ribosome-inactivating proteins. It appears that at least the tertiary structure of RIP active sites is conserved among Type I RIPs, bacterial RIPs, and A-chains of Type II RIPs. In many cases, primary structure homology is also found. Ready et al., J. Biol. Chem., 259(24):15252–15256 (1984) and other reports suggest that the two types of RIPs are evolutionarily related.
Type I plant ribosome-inactivating proteins may be particularly suited for use as components of cytotoxic therapeutic agents. A RIP may be conjugated to a targeting agent which will deliver the RIP to a particular cell type in vivo in order to selectively kill those cells. Typically, the targeting agent (e.g., an antibody) is linked to the toxin by a disulfide bond which is reduced in vivo allowing the protein toxin to separate from the delivery antibody and become active intracellularly. Another strategy for producing targeted cytotoxic proteins is to express a gene encoding a cytotoxic protein fused to a gene encoding a targeting moiety. The resulting protein product is composed of one or more polypeptides containing the cytotoxic protein linked to, for example, at least one chain of an antibody.
A variety of such gene fusions are discussed in Pastan et al., Science, 254:1173–1177 (1991). However, these fusion proteins have been constructed with sequences from diphtheria toxin or Pseudomonas aeruginosa exotoxin A, both of which are ADP-ribosyltransferases of bacterial origin. These protein toxins are reported to intoxicate cells and inhibit protein synthesis by mechanisms which differ from those of the RIPs. Moreover, diphtheria toxin and exotoxin A are structurally different from, and show little amino acid sequence similarity with, RIPs. In general, fusion proteins made with diphtheria toxin or exotoxin A have been immunogenic and toxic in animals, and are produced intracellularly in relatively low yield. Another strategy for producing a cytotoxic agent is to express a gene encoding a RIP fused to a gene encoding a targeting moiety. The resulting protein product is a single polypeptide containing a RIP linked to, for example, at least one chain of an antibody.
Because some RIPs, such as the Type I RIP gelonin, are primarily available from scarce plant materials, it is desirable to clone the genes encoding the RIPs to enable recombinant production of the proteins. It is also desirable to develop analogs of the natural proteins which may be easily conjugated to targeting molecules while retaining their natural biological activity because most Type I RIPs have no natural sites (i.e. available cysteine residues) for conjugation to targeting agents. Alternatively, it is desirable to develop gene fusion products including Type I RIPs as a toxic moiety and antibody substances as a targeting moiety.
The present invention also provides novel humanized or human-engineered antibodies and methods for producing such antibodies which may be conjugated or fused to various toxins. Such conjugations or fusions are useful in the treatment of various disease states, including autoimmune diseases and cancer.
There are several reports relating to replacement of amino acids in a mouse antibody with amino acids normally occurring at the analogous position in the human form of the antibody. See, e.g., Junghaus, et al., Cancer Res., 50: 1495–1502 (1990) and other publications which describe genetically-engineered mouse/human chimeric antibodies. Also by genetic engineering techniques, the genetic information from murine hypervariable complementarity determining regions (hereinafter referred to as “CDRs”) may be inserted in place of the DNA encoding the CDRs of a human monoclonal antibody to generate a construct encoding a human antibody with murine CDRs. See, e.g., Jones, et al., Nature, 321: 522–525 (1986).
Protein structure analysis on such “CDR-grafted” antibodies may be used to “add back” murine residues in order to restore lost antigen-binding capability, as described in Queen, et al, Proc. Natl. Acad. Sci. (USA), 86:10029–10033 (1989); Co, et al., Proc. Nat. Acad. Sci. (USA), 88: 2869–2873 (1991). However, a frequent result of CDR-grafting is that the specific binding acitvity of the resulting humanized antibodies may be diminished or completely abolished.
As demonstrated by the foregoing, there exists a need in the art for cloned genes encoding Type I RIPs, for analogs of Type I RIPs which may be easily conjugated to targeting molecules, and for gene fusion products comprising Type I RIPs, and especially wherein such gene fusions also comprise an humanized antibody portion.